This invention relates in general to earth boring bits, and in particular to a mining bit with features that reduce cutter drag and wear on bit legs.
In certain mining operations, blast holes are drilled to a depth of fifty feet or so for receiving explosives. A drill bit having three rotatable cutters or cones is commonly used. The bit is secured to the end of the drill pipe, which is rotated, causing the cutters to rotate and disintegrate the earth formation.
A typical bit has three head sections that are assembled with cutters, then welded together. A bit leg depends from each head section, and a bearing pin depends from each bit leg. A cutter with a central cavity is mounted on the bearing pin, with roller bearings placed between the cutter and the bearing pin. The bit legs and bearing pins contain passages for air to be pumped through from the drill pipe. Some of the air passes between the roller bearings for cooling, and discharges into the borehole for assisting in conveying cuttings into the surface.
The bearing pin is normally an integral component of the bit leg. At the intersection of the bit leg and bearing pin on the lower side, a depending flange, normally called the "shirttail", is formed. On the upper side of the bearing pin and on the inside surface of the shirttail, the intersection of the bearing pin with the bit leg is known as the "last machined surface". The cutter has a backface surrounding the central cavity entrance. A gage surface encircles the backface. When assembled, the backface is closely spaced from the last machined surface. The close spacing is believed to prevent cuttings from entering the bearings. The shirttail also serves to retain the roller bearings in the roller race, which is normally formed in the bearing pin.
One problem with the close spacing between the last machined surface and cutter backface results from the practice of running the bits until destruction. Eventually, thrust wear will push the cutter outward on the bearing pin, causing it to contact the last machined surface. The cutter will drag, damaging the cutting structure and shortening the life of the bit. Another failure point occurs because of the rotation of the bit leg against the borehole wall. This rotation causes wear on the shirttail, which may eventually wear through. The rollers would then fall out, causing the cutters to quickly lock.